A variety of congenital, acquired, or induced syndromes are associated with insufficient numbers of erythrocytes (red blood cells or RBCs). The clinical consequence of such syndromes, collectively known as the anemias, is a decreased oxygen-carrying potential of the blood, resulting in fatigue, weakness, and failure-to-thrive. Erythropoietin (EPO), a glycoprotein of molecular mass 34,000 daltons, is synthesized and released into the systemic circulation in response to reduced oxygen tension in the blood. EPO, primarily synthesized in the kidney and, to a lesser extent, in the liver, acts on erythroid precursor cells [Colony Forming Units-Erythroid (CFU-E) and Burst-Forming Units-Erythroid (BFU-E)] to promote differentiation into reticulocytes and, ultimately, mature erythrocytes.
The kidney is the major site of EPO production and, thus, renal failure or nephrectomy can lead to decreased EPO synthesis, reduced RBC numbers, and, ultimately, severe anemia as observed in predialysis and dialysis patients. Subnormal RBC counts may also result from the toxic effects of chemotherapeutic agents or azidothymidine (AZT) (used in the treatment of cancers and AIDS, respectively) on erythroid precursor cells. In addition, a variety of acquired and congenital syndromes, such as aplastic anemia, myeloproliferative syndrome, malignant lymphomas, multiple myeloma, neonatal prematurity, sickle-cell anemia, porphyria cutanea tarda, and Gaucher's disease, include anemia as one clinical manifestation of the syndrome.
Purified human EPO or recombinant human EPO may be administered to patients in order to alleviate anemia by increasing erythrocyte production. Typically, the protein is administered by regular intravenous injections. The administration of EPO by injection is an imperfect treatment. Normal individuals maintain a relatively constant level of EPO, which is in the range of 6-30 mU/ml, depending on the assay used. After typical treatment regimens, serum EPO levels may reach 3,000-5,000 mU/Ml following a single injection, with levels falling over time as the protein is cleared from the blood.
If a relatively constant level of EPO is to be provided in the blood (i.e., to mimic the normal physiology of the protein), a delivery system that is capable of releasing a continuous, precisely dosed quantity of EPO into the blood is necessary.